Flag flip for PageLifecycle.

I2S Link: https://groups.google.com/a/chromium.org/d/msg/blink-dev/Na7aTFJOpWs/_tpdtav8CAAJ
Plan is to flip the flag before branch cut and unflip if Intent doesn't go through.

Change-Id: I4564e4180b7903f67ac6186390b6c5d1acb74cb7
Reviewed-on: https://chromium-review.googlesource.com/1070448Reviewed-by: Ojan Vafai <ojan@chromium.org>
Reviewed-by: Fadi Meawad <fmeawad@chromium.org>
Commit-Queue: Shubhie Panicker <panicker@chromium.org>
Cr-Commit-Position: refs/heads/master@{#561382}

third_party/WebKit/LayoutTests/virtual/stable/webexposed/global-interface-listing-expected.txt

@@ -1216,6 +1216,7 @@ interface Document : Node
     getter onended
     getter onerror
     getter onfocus
+    getter onfreeze
     getter ongotpointercapture
     getter oninput
     getter oninvalid
@@ -1254,6 +1255,7 @@ interface Document : Node
     getter onreadystatechange
     getter onreset
     getter onresize
+    getter onresume
     getter onscroll
     getter onsearch
     getter onseeked
@@ -1288,6 +1290,7 @@ interface Document : Node
     getter title
     getter visibilityState
     getter vlinkColor
+    getter wasDiscarded
     getter webkitCurrentFullScreenElement
     getter webkitFullscreenElement
     getter webkitFullscreenEnabled
@@ -1386,6 +1389,7 @@ interface Document : Node
     setter onended
     setter onerror
     setter onfocus
+    setter onfreeze
     setter ongotpointercapture
     setter oninput
     setter oninvalid
@@ -1424,6 +1428,7 @@ interface Document : Node
     setter onreadystatechange
     setter onreset
     setter onresize
+    setter onresume
     setter onscroll
     setter onsearch
     setter onseeked

third_party/blink/renderer/platform/runtime_enabled_features.json5

@@ -911,7 +911,7 @@
     },
     {
       name: "PageLifecycle",
-      status: "experimental",
+      status: "stable",
     },
     {
       name: "PagePopup",